The invention resides in a quantity-limiting valve for a fuel injection system of an internal combustion engine, the valve including a cylinder with a piston dividing the cylinder interior into an inflow region and an outflow region which, regions are in communication via a communication channel extending along the cylinder which is axially movable for controlling the fuel flow quantity passing through the valve.
Quantity limiting valves of the type with which the present invention is concerned are known. Generally such a quantity limiting valve is arranged in a conductor extending between a high pressure fluid source and an injector in order to limit the fuel quantity supplied to the injector during an opening cycle, and, in this way, the fuel quantity which can be injected into a combustion chamber of the internal combustion engine. In this way, damage to the internal combustion engine by an excessive amount of fuel injected in the combustion chamber for example via a defective injector which may no longer properly close or not close at all, can be prevented.
Such a quantity limiting valve generally includes a housing with an inflow region and an outflow region. It further includes a piston which is movably disposed in a cylinder. The piston divides the cylinder into the inflow region and the outflow region, wherein a fluid communication path is provided via a transfer flow passage extending between a circumferential surface area of the piston and an inner surface area of the cylinder. In a first operating position the piston is biased to abut with its front surface a contact surface of a stop element. The quantity limiting element is arranged in flow direction between the high pressure source and the injector or it is integrated into the injector upstream of the injection structure. As long as the injector is closed, that is a fluid communication to a combustion chamber to which the injector is assigned is blocked, the quantity limiting valve is in its first operating position. When the injector is opened, fuel flows out of the outflow region into the combustion chamber. As a result, the pressure in the outflow region drops and a pressure differential between the inflow region and the outflow region across the piston develops. Because of this pressure differential, the piston is lifted off the stop element and is displaced into the outflow region. At the same time, fuel flows via the transfer passage from the inflow region to the outflow region. The flow cross-section of the transfer passage is so selected that less fuel can flow per time unit via the transfer passage from the inflow region to the outflow region than is supplied to the combustion chamber via the injector from the outflow region. As a result, the pressure difference between the inflow area and the outflow area remains and the piston is moved further toward the outflow region as long as the injector is open. When the injector is closed, fuel continues to flow out of the inflow area via the transfer passage into the outflow region whereby the pressure differential decreases as the piston is moved back into the inflow region until its front area abuts again the stop element where it is again in its first operation position.
However, if the injector remains open because of a defective injector, the piston is moved further into the outflow region and into contact with a sealing surface which it sealingly abuts. Here, the piston is in its second operating position in which the inflow region is fluidically separated from the outflow region or at least from an outflow area of the injector from which fuel is injected into the combustion chamber. Then, fuel can no longer flow from the inlet region to the outlet region. The outlet region is then open to the combustion chamber via the defective injector whereby the pressure differential across the piston is maximized. The piston therefore remains pressed against the sealing surface which blocks any fuel from entering the combustion chamber so that the internal combustion engine is effectively protected from damage by an excessive fuel amount supply to the combustion chamber.
The known quantity limiting valve has the disadvantage that, at the beginning of an injection, the release of the piston out of its first operating position is delayed but then the piston lifts off suddenly from its seat in the first operating position. In particular, if the pressure curve in an individual storage assigned to a particular injector is used for determining the injection begin, the sudden lift off of the piston results in an opening wave superimposed to the pressure curve, that is, to a temporary local excessive pressure which causes an erroneous evaluation of the pressure curve and consequently a faulty determination of the injection begin. It is noted that the opening wave typically changes over the life of the quantity limiting valve. Therefore, faulty evaluation of the pressure signal detected in the individual storage area are unavoidable.
It is therefore the object of the present invention to provide a quantity limiting valve which does not have the disadvantages described above. In particular the quantity limiting valve should avoid the sudden lifting of the piston from its seat in the first operational position so that no excess pressure opening wave occurs, that is, no pressure signal should occur in an individual storage assigned to an injector so that this pressure signal provided in an individual storage area can be evaluated free of errors in a reproducible way and, in particular, an injection begin can be accurately determined.